1. Field of the Invention
This invention relates to the detection of ionizing radiation and specifically to an improved apparatus for the detection of radon gas using a track registration material.
2. Prior Art
Measurement of radon gas concentrations by the technique employing track registration material is a known technique and has been found to be effective. One example is the method and apparatus for detecting radon and alpha emitting daughters of radon disclosed and claimed in U.S. Pat. No. 3,665,194 to Alter, et al. for "Radon Detection", the disclosure of which is hereby incorporated by reference. According to this technique, an apparatus containing a track registration material is employed to monitor the presence of alpha particles emitted from radon and radon decay products. If the track registration material is irradiated by alpha particles having sufficient energy to penetrate the surface of the track registration material, minute damage "tracks" are created therein, due to damage of the registration material by the impinging alpha particles. These tracks can be enlarged by contact with a reagent to which the tracks display preferential chemical reactivity as compared to areas apart from the tracks and made visible under magnification.
This radon monitoring technique is usually carried out by placing an apparatus containing a strip of track registration material, in a location to be monitored (such as a dwelling), and permitting the apparatus to remain at the monitored site for a preselected period of time (e.g. several weeks to one year), during which alpha tracks are formed in proportion to the concentration of alpha particle emitting substances. After exposure, each strip of track registration material is subjected to a chemical etching solution, and the number of track etched per unit area on the strip can be counted by microscopic inspection or by electronic scanning techniques. The obtained tracks per unit area data is then used to compute the average radon concentration at the monitored site.
In past applications, efforts have been directed toward designing special track registration detectors dedicated to a particular application. For examples, in U.S. Pat. No. 4,338,523 issued July 6, 1982, for "Low-Cost Track Registration Radiation Detector", a track registration detector is disclosed which is designed specifically for use in monitoring radon levels in interior structures; and in U.S. Pat. No. 4,518,860 issued May 21, 1985, for "Compact Detector for Radon and Radon Daughter Products, another monitor with physical geometry requirements is described which has certain and specific physical geometry requirements. As a result of these requirements, there is a design impediment which requires a rigid and specific geometry for the apparatus housing and the strip location for adequately recording exposure to alpha particles.
This invention also relates to short term monitoring, which heretofore has not been generally performed with alpha track registration material, wherein the objective is monitoring over a period of two days to seven days. Application of track registration material to short term monitoring has not been generally made due to the lack of statistical significance of results at very low exposures and/or over only a short period of time.
Many closed containers perform adequately as long as there is standardization of the particular configuration within the housing and only radon gas and not the radon daughter products are permitted to enter. The best statistical significance is obtained by a large number of tracks. The larger the area analyzed yields a larger number of tracks regardless of the concentration or exposure period. Statistical significance also requires that for small concentrations near the one picocurie per liter level and/or short periods of exposure that any imperfections on the strip's surface which resemble tracks be consistently low in number. The measurement uncertainty depends on the number of tracks counted, variations in track registration material, and the calibration factor. The strips with the indicated and desired quality as stated are commercially available and exposure time requirements can be reduced to two days but may also be allowed for up to one year. An effective track readout system is a microfiche reader which performs adequately for review and combined with a printer, the microfiche allows hard copy images for ease of viewing large areas.
The current suppliers have required placement of the detectors for a minimum of several weeks. Impediments have also occurred due to one or a combination of the following limitations:
(1) a rigid or bulky housing;
(2) the requirements of the physical size of the track registration material used in each detector;
the requirements for the devices to be boxed for shipment or storage;
(4) the requirement for a separate data information form for the user to maintain; and,
the need for higher postage fees because of the weight of the device and shipping box.
Thus, a need has existed for a track registration type alpha particle detector which is less expensive to use and which can be used effectively and interchangeably in a wider variety of radon monitoring applications.